feat(CI): add a GitHub Action to lint the Markdown

Ignore `mruby-bin-config` in cross-building

Run on pull request only
Using https://www.npmjs.com/package/markdownlint-cli
Lint Markdown for rules:
- MD009/no-trailing-spaces
- MD012/no-multiple-blanks
- MD022/blanks-around-headings
- MD031/blanks-around-fences
- MD032/blanks-around-lists

🔒 Fix missing HTTPS on links

🔒 Fix missing HTTPS on links

This is because `MRuby::Build.currrent` and `MRuby::CrossBuild.current` are assignments to different instance variables.

feat(CI): add a GitHub Action to check spelling

- Fix spelling
- Run only on pull request
- Using https://github.com/client9/misspell

feat(CI): add a GitHub Action to lint the YAML

Rename build configuration files for CI

Currently, there are build configuration files for CI, `travis.rb` and
`appveyor.rb`, but they are used for GCC/Clang and MSVC, not for Travis CI
and Appveyor, respectively.

Therefore, rename them to `gcc-clang.rb` and `msvc.rb`, respectively, and
move them under `build_config/ci/` to clarify that they are for CI.

Guess toolchain when `MRuby::Build#toolchain` argument is omitted

- Run only on pull request
- Add a `.yamllint` config file
- Lint YAML

Ensure initialization of `RVALUE_zero` in `mrb_obj_alloc`

Output build log at the time of installation

refactor: remove trailing whitespace from C, Header, Ruby and YAML files

Lint

Union initialization initializes the first member. The first member of
`RVALUE` is `struct free_obj`, but because it is only 4-words, it seems that
initialization after the 5th word is not ensured.

Therefore, I created 6-words `struct RVALUE_initializer` for initialization
and made it the first member.

Fix spelling

```ruby
def method_missing(meth, ...)
  send(:"do_#{meth}", ...)
end
```

They used to be `size_t`, `uint32_t` and `ptrdiff_t`. Now all of them
made to be `uint32_t`.

Merge pull request #5217 from shuujii/call-super-before-creating-host-build-in-MRubyCrossBuild-initialize

Call `super` before creating `host` build in `MRuby::CrossBuild#initialize`

Call `super` (`block`) ahead so that creation of the `host` build can be
skipped if pre-built `mrbc` can be specified in the future.

close #5213.

Allow obsoluted `MRB_METHOD_T_STRUCT`

Remove unused `@endian` in `MRuby::CrossBuild#initialize`

Update "Configuration Options Changed" in `doc/mruby3.md` [ci skip]

Make it possible that `libmruby.a` is not created